1. Field of the Invention
The present invention relates to a light-emitting diode device having a long service life, and an encapsulant used in this light-emitting diode device.
2. Description of the Related Art
Resin encpsulation of semiconductor devices has been practiced to maintain the quality and facilitate the handling of the devices. Many kinds of resin have been used as transparent encapsulants for light-emitting diodes (to be referred to hereinafter as LEDs), typical example thereof being an epoxy resin composition containing an acid anhydride curing agent and a curing accelerator.
Published Unexamined Japanese Patent Application Nos. 59-62624 and 60-140884 disclose epoxy resin compositions by adding, respectively, thiophosphite compounds and thio-phosphoric acids to improve transparency.
An internal stress occurs in these encapsulants, generating a strain which adversely affects the semiconductor elements encapsulated therein by causing internal disconnections and decreases in luminance. In particular, Infrared Ray (IR) and yellow LEDs tend to be adversely affected by this distortion and stress.
An attempt is made to obtain a low stress encapsulant as a means for solving the above problem.
The following methods are available to decrease the stress of the encapsulant:
(1) A method for decreasing the glass transition temperature (Tg) of a resin. This method includes techniques for making an encapsulant flexible and a technique for decreasing the crosslinking density of a resin.
(2) A method wherein a resin composition is prepared such that the coefficient of linear expansion c is decreased and is used. This method includes a technique for adding a filler to a resin.
(3) A method for decreasing the modulus of elasticity (Young's modulus of elasticity) E. This method includes a technique for obtaining a "sea and island" structure of resins.
(4) A method for decreasing the shrinkage factor .epsilon. during the curing of a resin. This method includes a technique for performing a reaction so as to cause uniform progress of resin curing.
An increase in the flexibility and a decrease in the crosslinking density of an encapsulant cause degradation of the heat resistance of the resin and an increase in the coefficient of linear expansion .alpha.. When a filler is added to a resin, the transparency of the resin is degraded, and its modulus of elasticity E is increased. In addition, when a "sea and island" structure of the resins is obtained, its heat resistance is decreased. Furthermore, to cause a uniform curing reaction of the resin is to greatly degrade workability.